Retinal function camera

ABSTRACT

A retinal camera is used to examine an eye, the camera including a light source having first and second sources emitting first and second wavelength bands. The first and second light sources are arranged to alternately produce light onto the retina such that the absorptivity of light of the first wavelength band by oxygenated blood is greater than the absorptivity of light of the second wavelength band, and the absorptivity of light of the first wavelength band by the oxygenated blood is less than absorptivity of light of the second wavelength band. Light is selectively focused from the first and second sources by an optical arrangement and imaging devices produce respective images of a portion of the retina illuminated with the respective wavelength bands. The images obtained by the imaging device are processed by the imaging device and processor to determine a retinal metabolic image based on haemoglobin oxygenation. In an embodiment of the invention light is scanned across the retina.

CROSS REFERENCE TO RELATED APPLICATION

This application is a U.S. National Phase of International PatentApplication serial No. PCT/GB02/01538, filed Apr. 3, 2002 which claimspriority of Great Britain Patent Application serial No's. 0108885.5,filed Apr. 9, 2001, 0111975.9, filed May 16, 2001 and 0119155.0, filedAug. 6, 2001.

This invention relates to a retinal function camera.

Age-related macular degeneration may cause loss of macula function of aneye due to death of photoreceptor calls and retinal pigment epithelium.This results in the gradual loss of detailed central vision. Inaddition, small yellow deposits in the centre of the retina, known asdrusen, are seen in the early stages of macular degeneration. Peopleaged over 50 who have drusen are at risk of developing choroidalneovascularisation. This refers to small new abnormal blood vessels thatappear to form in response to tissue hypoxia. In such neovascularage-related macular degeneration, the abnormal new blood vessels fromthe choroidal layer grow and proliferate with fibrous tissue within thedrusen material. This choroidal neovascularisation may cause acute lossof vision as transudate or haemorrhage accumulates within or beneath theretina. The transudate, haemorrhage or scar tissue may be seen onophthalmoscopy but fluorescein angiography may be needed to visualisethe abnormal blood vessels. The area of choroidal neovascularisation maybe treated by either laser photocoagulation or, if the new vesselsextend under the centre of the retina, by photodynamic therapy.

However, the new blood vessels are difficult to see. Screening for thechoroidal new vessels and their complications, which may develop over ashort time, is currently done by identifying loss of vision. Thediagnosis and assessment requires investigation by an ophthalmologist,who may need to use fluorescein angiography to see the new choroidalvessels. Current screening for choroidal neovascularisation involves apatient observing straight lines on a piece of graph paper and reportingany distortion of the lines or the development of blank spots.

Alternatively, any change of retinal metabolism, such as maculardegeneration and diabetic retinopathy, may be assessed by studies of theoxygenation of blood in the retina. Arterial blood is highly oxygenatedwhile venous blood is deoxygenated. Areas of retinal tissue hypoxia maybe recognised before the development of new vessels.

The oxygenation of blood in the retina can be determined by illuminatingthe blood with infrared light of different wavelengths due todifferential absorption of different wavelengths by oxygenated anddeoxygenated blood. Deoxygenated blood illuminated at 760 nm appearsdarker than when it is illuminated at 1000 nm. Conversely oxygenatedblood illuminated at 760 nm appears lighter than when illuminated at1000 nm. In illumination at both 760 nm and 1000 nm partiallydeoxygenated blood appears on a grey scale.

It is known from U.S. Pat. No. 4,877,322 to use this property to measurerelative oxygen saturation of choroidal blood of the eye fundus and moreparticularly to make such measurements in specifically selected areas ofthe eyegrounds for the study of glaucoma and macular degeneration. Inthis prior art disclosure the retina is illuminated simultaneously withwhite, red and infrared light and the relative absorption of red andinfrared light used to determine the oxygenation and, hence, theconcentration of capillaries in regions of the retina. However, becausethe retina is illuminated with all three wavelengths simultaneously, itis not possible to obtain any detailed view of retinal function.

It is known from U.S. Pat. No. 5,219,400 to determine the degree ofhaemoglobin oxygenation in the blood vessels of the retina underconditions of dark-adaptation and light-adaptation by directing a beamof near-infrared light having a range of wavelengths from 700–100 nm ata blood vessel in the retina, measuring the intensity of thebackscattered light from the blood vessel in the range from 700 to 800nm at regularly spaced intervals of wavelength such as 2 nm, anddetermining the degree of haemoglobin oxygenation by reference to acorrelation between haemoglobin oxygen and light absorbance in thenear-infrared spectral range. There is also disclosed an artificial eyemodel for calibration of haemoglobin oxygen saturation and near infraredreflective spectral data. However, there is no disclosure of theformation of an image of retinal function.

U.S. Pat. No. 5,400,791 discloses the use of infrared laser lightbetween 795 nm and 815 nm for angiography.

U.S. Pat. No. 6,244,712 discloses the use of sequential scan linesilluminated with alternate lasers to form an interlaced data frame, andthe use of an r-wave of an electrocardiogram to trigger laserillumination of a retina.

It is an object of the present to at least mitigate the foregoingdifficulties.

According to a first aspect of the invention there is provided a retinalfunction camera comprising: a first source of light of a firstwavelength band; a second source of light of a second wavelength band,the absorptivity of light of the first wavelength band by oxygenatedblood being greater than the absorptivity of light of the secondwavelength band and the absorptivity of light of the first wavelengthband by deoxygenated blood being less than absorptivity of light of thesecond wavelength band; means for focusing light selectively from thefirst and second sources on a portion of a retina of an eye; imagingmeans for producing respective images of a portion of the retinailluminated with the respective wavelength bands; and processing meansadapted to process the respective images obtained by the imaging meansto determine isoreflective points of the respective images at whichabsorption of light of the first wavelength is substantially equal toabsorption of light of the second wavelength and areas of the respectiveimages having differential absorptivity for the first and secondwavelengths, to obtain a retinal function image based on haemoglobinoxygenation.

Conveniently, the processing means comprises means for displaying therespective images alternately, at a predetermined frequency, such thatthe areas of the respective images having differential absorptivity atthe first and second wavelengths, flicker.

Preferably, the predetermined frequency is 12 Hz.

Advantageously, the first and second wavelength bands are selectedbetween 488 nm and 1000 nm, to produce a functional image.

Conveniently, the first wavelength band is centred substantially on 830nm and the second wavelength band is centred substantially on one of 635nm and 670 nm and 760 nm; or the first wavelength band is centredsubstantially on 910 nm and the second wavelength band is centredsubstantially on one of 635 nm and 670 nm and 760 nm.

Conveniently, there is provided an array of superluminescent diodesproducing light in the wavelength band 550 to 650 nm to produce aconventional image and the second wavelength band is 700 nm to 805 nmand the first wavelength band is 805 nm to 1000 nm to produce afunctional image.

Advantageously, the processing means comprises means for assigning therespective images created with the first wavelength band and the secondwavelength band and the conventional image with false coloursrespectively and combining the three images to form a combined colourimage.

Conveniently, the first source of light is a laser.

Advantageously, the second source of light is a laser.

Conveniently, the first and second sources of light are superluminescentdiodes provided with narrow band pass filters to restrict the wavebandof light emitted.

Alternatively, a wide spectrum light source is provided wavelengths fromnear infrared through the visible spectrum and the first and secondlight sources are produced by passing the wide spectrum light throughnarrow band pass filters.

Conveniently, the means for focusing light selectively from the firstsource and the second source comprise means for focusing light from thefirst and second sources and for sequentially switching on and off thefirst and second sources alternately.

Alternatively, the means for focusing light selectively from the firstand second sources comprise means for focusing light from the first andsecond sources and shutter means for alternately interrupting light fromthe first and second sources, respectively.

Conveniently, the processing means includes means for comparing an imagewith a reference image formed at an earlier time.

Preferably, the processing means includes pattern recognition means foraligning the image with the reference image.

Advantageously, the processing means includes means for assigning firstareas of the respective images having differential absorptivity for thefirst and second wavelets corresponding to portions of the retina havinggreater oxygenation than the isoreflective points a first false colourand for assigning second areas of the respective images havingdifferential absorptivity for the first and second wavelengthscorresponding to portions of the retina having less oxygenation than theisoreflective points a second false colour and for generatingintensities of the false colours at each point in the image proportionalto the difference in oxygenation of that respective point from theoxygenation of the isoreflective points.

Preferably, a wavelength or power of the first source and/or the secondsour is variable to tune the isoreflective point.

Conveniently the processing means includes means for calibratingoxygenation by identifying a portion of the retina image of maximumoxygenation and a portion of the retina image of maximum de-oxygenation.

Advantageously, the means for focusing light include scanning means forscanning the focused light across at least a portion of the retina.

Conveniently, the scanning means include first scanning means forscanning the focused light horizontally across the at least a portion ofthe retina and second scanning means for scanning the focused lightvertically across the at least a portion of the retina.

Preferably, the first scanning means includes one of a rotatablepolygonal mirror and a vibratable plane mirror.

Advantageously, the second scan means includes a galvanometer scanner.

Conveniently, first synchronising means are provided to synchronise thefirst and second scanning means with selection means for selectivelyoperating the first source of light and the second source of light.

Advantageously, second synchronising means are provided to synchronisethe first and second scanning means with the imaging and processingmeans.

Conveniently, the first and second scanning means are adapted to de-scanlight reflected from the retina and reflecting de-scanned light to theimaging and processing means.

Advantageously, the first and second scanning means operate atfrequencies corresponding to television scanning frequencies such thatthe imaging and processing means may be used to form a television image.

Conveniently, at least one of a confocal filter is provided locatableupstream of the imaging and processing means for detecting a retinalsurface image and for blocking a deeper choroidal image and ananti-confocal filter locatable upstream of the imaging and processingmeans to block the retinal surface image and allow the deeper choroidalimage to be detected.

Advantageously, a first linear polarising filter is provided between thelight source and the eye and a second poling filter orthogonal to thefirst linear polarising filter is provided between the eye and theimaging and processing means such that the second orthogonal polarisingfilter blocks light reflected from a surface of the eye.

Conveniently, an optical beam adder is provided for allowing a firstlaser beam from the first light source and a second laser beam from thesecond light source access to an optical axis of the retinal functioncameras.

Preferably, the processing means includes an imaging device sensitive tolight emitted by the first source of light and the second source oflight.

Conveniently, the imaging device is one of a CMOS array, a CCD array, aphotodetector or an infrared image sensor.

Advantageously, optical fibre and lens means re provided for producing apoint source of light from light from the first source of light andlight from the second source of light.

According to a second aspect of the invention, there is provided amethod of obtaining a retinal function image based on haemoglobinoxygenation, the method comprising the steps of providing a retinalfunction camera having a first source of light of a first wavelengthband and a second source of light of a second wavelength band, theabsorptivity of light of the first wavelength band by oxygenated bloodbeing greater than the absorptivity of light of the second wavelengthband and the absorptivity of light of the first wavelength band bydeoxygenated blood being less than the absorptivity of light of ticsecond wavelength band; focusing light selectively from the first andsecond sources on a portion of a retina of an eye; producing respectiveimages of the portion of the retina illuminated with the respectivewavelength bands; and processing the respective images to determineisoreflective points of the respective images at which absorption oflight of the first wavelength is substantially equal to absorption oflight of the second wavelength and areas of the respective images havingdiffer absorptivity for the first and second wavelengths to obtain aretinal function image based on haemoglobin oxygenation.

Conveniently, the step of processing the images comprises: assigning tofirst portions of the image corresponding to portions of the retinahaving greater oxygenation than the isoreflective point a first falsecolour and generating intensity of the false colours at each point inthe image proportional to the flicker contrast that is proportional tothe difference in oxygenation of that respective point from theoxygenation of the isoreflective point, assigning to second portions ofdie image corresponding to portions of the retina having lessoxygenation than the isoreflective point a second false colour andgenerating intensity of the false colours at each point in the imageproportional to the flicker contrast that is proportional to thedifference in oxygenation of that respective point from the oxygenationof the isoreflective point, assigning a third colour to theisoreflective point, and constructing a composite image by combining thefirst, second and third colour age data to form a colour image ofretinal function based on haemoglobin oxygenation data.

Preferably, the first colour is red.

Preferably, the second colour is blue.

Preferably, the third colour is yellow.

Specific embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 shows a first embodiment of the invention in schematic form;

FIG. 1A shows a cross-section along the double arrow headed line 1A—1Aof FIG. 1;

FIG. 2 shows a second embodiment of the invention in schematic form;

FIG. 2A shows a cross-section along the double arrowhead lines 2A—2A ofFIG. 2; and

FIG. 3 shows a third embodiment of the invention in schematic form.

In the diagrams, like reference numerals-denote like parts.

Referring to FIG. 1, a retinal camera 101 is used to examine an eye 10.The camera includes a light source 1 comprising an integrating sphere 20having first and second sources 21,22 disposed at substantially 120° toeach other, emitting first and second infrared wavelength bandsrespectively and a visible light source 23, to provide diffuseillumination. The primary function of the integrating sphere is toproduce alternating infrared light from the same point source so thatthe retinal images, formed by the two infrared sources 21,22, arealigned. The 600 nm visible light source 23 is useful in differentiatingveins and arteries in the retinal image. It is present to allow afunctional image obtained with the infrared sources to be compared witha conventional image.

The functional image may be generated by contrasting images obtainedfrom both the visible (450–700 nm) and infrared (700–1000 nm) spectra.The infrared haemoglobin oxygen saturation light absorption is welldefined-between 700–805 nm and 805–1000 nm. The visible spectrum has acomplex haemoglobin light absorption relationship as disclosed in VanAssendelft OW. Spectrophotometry of haemoglobin derivatives. RoyalVangorcum, Assen, The Netherlands: Thomas, 1970. Blue light between 450nm and 500 nm may be contrasted with red light between 600 nm and 700 nmto generate a functional image. The blue light in the range 450–500 nmmay also be contrasted with the near infrared 700–805 nm light.

Thus, light between 450 nm and 1000 nm may be used. Suitable pairs ofwavelengths are: 488 nm and 600 nm or 635 nm or 670 nm or 760 nm; or 635nm and 830 nm or 910 nm; or 670 nm and 830 nm or 910 nm; or 760 nm and830 nm or 910 nm.

Possible sources of illumination include an array of superluminescentdiodes, producing light in the range 550 nm to 650 nm, and preferably at600 nm, to produce a conventional image and superluminescent diodessequentially illuminating a diffuse reflective integrator sphere withinfrared light in the region of 758 nm (700 nm–805 nm) and 910 nm (805nm–1000 nm) to provide illumination for a functional image. Narrowbandpass filters may be used with the superluminescent diodes torestrict their bandwidth. Alternative optical arrangements for the lightsource include a beam splitter arrangement for the near infraredsuperluminescent diodes. The superluminescent diodes may either switchon and off sequentially or their light be sequentially blocked with ashutter. Alternative light sources may be used. For example a widespectrum source emitting radiation from near infrared through thevisible spectrum such as a xenon light doped with other gases to providea near infrared spectrum between 700 nm to 1000 nm with narrow band passfilters may be used. Alternatively, laser diodes may be used as theinfrared sources, in which case to avoid the speckle effect of laserlight, the integrating sphere 20 converts collimated, coherent, narrowband light from the laser diodes into uncollimated, incoherent, narrowband light. It will be understood that alternative apparatus forproducing a point source of light formed from the first source of lightand second source of light may be used, for example, an optical fibreand lens apparatus may replace the integrating sphere, and dichroic beamcombiner or half silvered mirror.

Light from the light source is collimated by a condenser lens 30 andpassed through an annular ring diaphragm 40 before being reflected by amirror 50 and passing through a relay lens 60. A cross-section of theannular ring diaphragm is shown in FIG. 1A, in which is shown an annulartransparent portion 41 within an opaque support 42. A cone of lightemergent from the annular diaphragm and reflected by the mirror 50 isthen reflected by a perforated mirror 80, having a central transmissionhole 81, through an objective lens 90 into the eye 10 to produce anevenly illuminated area at a focal plane of the eye 10. An internalfixation target 70 is provided between the relay lens 60 and theperforated mirror 80, on an optical axis defined thereby. The internalfixation target 70 may be a small illuminated object such as cross-wireson which the eye 10 may be focussed. After absorption within the retina,light is reflected from the retina out of the eye back through theobjective lens 90 and a portion of the reflected light passes throughthe central aperture 81 of the perforated mirror 80, and sequentiallythrough an occluding diaphragm 110, a focus lens 120 and an imaging lens130 to form an image in an image recorder and processor 140. The annularrig diaphragm 40 and the pupil of the eye 10 are arranged in conjugatepositions in the illuminating optical system and the pupil of the eye10, transmission hole 81 of the mirror 80 and the aperture 111 of theoccluding diaphragm 110 are arranged in conjugate positions of theobjective optical system.

FIG. 2 illustrates a further embodiment 102 of the invention in whichthe integrating sphere of the first embodiment shown in FIG. 1 isreplaced by a half silvered mirror 24 and two superluminescent diodeswith narrow band pass filters 21′,22′ are disposed so that infraredlight of a first waveband from the first superluminescent diode 21′passes through the mirror along the optical axis 11 and infrared lightof a second waveband from the superluminescent diode 22′ is reflected bythe half silvered mirror 24 to also pass along the optical axis 11.Other parts of the embodiment are as described for the first embodimentillustrated in FIG. 1.

The operation of the two embodiments of the invention is similar. Takingthe second embodiment of FIG. 2 as an example, with the patient focusingthe eye 10 to be studied on the fixation target 70 the pupil of the eyeis located adjacent to the aspheric ophthalmic objective lens 90. Thisaligns the pupil and the foveola to ensure that when the light source isactivated to illuminate the retina, light is transmitted through thepupil rather than reflected from the iris. Without an illuminated objectthe eye would wander while looking into a black void. The illuminatedobject is faint in intensity to avoid pupil constriction. A typicalilluminated object is a fine cross or concentric circle cut out of anopaque screen in front of a low-powered light-emitting diode. Analternative would be illuminated cross-wires.

Either visible 600 nm light from a superluminescent diode or xenon lightsource (not shown) is used to obtain a conventional image. Alternatinginfrared illumination in the region of 758 nm and 910 nm is provided bythe superluminescent diodes 21′,22′ to project infrared light beams ontothe retina to obtain a functional image.

The image recorder and processor 140 comprises an imaging devicesensitive to light in the designated spectrum, for example a CMOS or CCDarray, photodetector, infrared or visible light sensor, or otherinfrared image sensor.

In order to analyse the images, account must be taken of residualinternal reflection inside the optical system. This is minimised by ablack absorptive internal surface and the use of ridging or internalbaffles. In addition, the light output from the light source may bevariable and light is absorbed by the front surface coatings of themirrors and the lenses. Further, light may leak into the system fromaround an eye seal which may produce a light flare and in some patients,vasoconstriction due to drugs and smoking may alter the retinaloxygenation. These problems may be largely obviated by comparing theimages produced under different wavelength illuminations or at differenttimes.

A visualisation of altered retinal function or structure may be obtainedby comparing individual retinal field images with initial referenceretinal field images to detect any change. This involves the use ofpattern recognition software to obtain a “best fit” superimposition ofthe reference and new image. The reference image is then subtracted fromthe new image leaving the components that have changed. The componentsthat have changed are then superimposed on the new image and identifiedby, for example, a colour change or flashing image. Alternatively, thethree images obtained with the visible light and two infraredwavelengths from the superluminescent diode sequential illumination maybe superimposed, after allocating each image a false colour (forexample, red, green and blue), to create a colour image.

As indicated above, deoxygenated blood illuminated at 760 nm appearsdarker than when it is illuminated at 1000 nm. Conversely oxygenatedblood illuminated at 760 nm appears lighter than when illuminated at1000 nm. In both images partially deoxygenated blood would appear on thegrey scale.

Therefore, if alternating images are displayed, at for example 12 Hz, ona screen most of the images of blood vessels will flicker but there willbe areas of blood vessels that do not flicker in light intensity. Thesenon-flickering blood vessels at the isoreflective point for equal energyillumination form a reference deoxygenation point. The non-flickering,isoreflective areas may be displayed in yellow. Areas that flicker havea significant difference in oxygenation from the isoreflective point.The greater the contrast of flicker the more saturated or desaturatedthe blood is with oxygen. The desaturated blood may be displayed in blueand the colour intensity related to the flicker contrast. The oxygenatedblood may be displayed in red and the colour intensity related to theflicker contrast. This produces a subjective image of retinal functionwith one isoreflective point.

The isoreflective point, of a composite image formed by the first andsecond light sources, may be tuned by varying the wavelength or power ofeither or both of the light sources.

To determine absolute values of oxygenation it is necessary to calibratethe image. A retinal artery with maximum flicker contrast whenilluminated with 900 nm–1000 nm near infrared light is examined. Aninspired oxygen concentration FiO₂ is increased from 21% to, forexample, 50% to ensure that the retinal artery blood is 100% saturated.This provides a reference for 100% oxygen saturation of retinal blood. Aretinal haemorrhage with maximum flicker contrast when illuminated withnear infrared light in the region of 760 nm is examined. The retinalhaemorrhage consists of deoxygenated blood. The inspired oxygenconcentration FiO₂ may be decreased from 21% to 10% to ensure that thereis no increase in flicker contrast or the FiO₂ may be increased to 50%to ensure that there is no reduction in flicker contrast. This providesa reference for deoxygenated retinal blood. Alternative calibration maybe obtained by perfusing either an animal eye or artificial eye modelwith haemoglobin of known oxygen saturation and recording infraredimages. This technique may be used to obtain a haemoglobin oxygensaturation level for the isoreflective point. The retinal functioncalibration techniques outlined may be repeated for cytochrome a,a₃,with infrared light in the range 700 nm to 1300 nm, to obtain a furtherisoreflective point and additional calibration. Cytochrome a,a₃ is usedin addition to haemoglobin oxygen saturation to assess tissueoxygenation states. However, the longer wavelengths used would havegreater tissue penetration and a possible degraded image quality.

Alternatively, an artificial eye model with three channels of blood maybe used for haemoglobin oxygen calibration. The eye model has a firstreference channel containing 50% oxygen saturated blood, a secondreference channel containing 100% oxygen saturated blood and a thirdassessment channel containing blood with variable haemoglobin oxygensaturation.

FIG. 3 illustrates a scanning laser retinal function camera which is athird embodiment of the invention. This produces sequential images ofthe fundus from two lasers 26,27.

The scanning laser retinal function camera 103, produces sequential nearinfrared images of the fundus required to generate the functional image.The camera comprises a multiple near infrared laser source that is ableto direct a narrow beam of infrared laser light in the region of 758 nm(700 nm–805 nm) via a mirror system and focus the light onto the fundus.The light reflected from the fundus is directed to an infrared detector,which produces an electrical output proportional to the intensity of thedetected infrared light. By moving the mirror system according to ascanning sequence in a raster fashion and synchronising the detector tothe scanning sequence, it is possible to produce an image of the fundus.The electrical output from the infrared detector is processed to displayan image of a portion of the fundus. The laser light in the region of758 nm (700 nm–805 nm) is then switched off and a narrow beam ofinfrared light in the region of 910 nm (805 nm–1000 nm) is focussed viathe mirror system onto the fundus. The infrared light in the region of910 nm which is reflected from the fundus is directed to the infrareddetector which produces an electrical output proportional to theintensity of the detected light. The electrical output is processed todisplay an image of a portion of the fundus.

The two images obtained are stored and then processed to be displayedalternately, at a predetermined frequency, to form a composite imagesuch that areas that have a differential absorptivity at the 700 nm–805nm and 805 nm–1000 nm wavelengths flicker. Non-flickering, isoreflectiveblood vessels contain partially oxygenated haemoglobin at which theabsorption of light from the 700 nm–805 nm laser is equal to theabsorption of light from the 805 nm to 1000 nm laser. The non-flickeringisoreflective areas may be displayed in yellow. Blood vessels thatflicker have a significant difference in oxygenation from theisoreflective point. The greater the contrast of the flicker the moresaturated or desaturated the blood is with oxygen. The desaturated bloodmay be displayed in blue and the colour intensity related to the flickercontrast. The oxygenated blood may be displayed in red and the colourintensity related to the flicker contrast. This produces a subjectivescanning laser image of retinal function with one isoreflective point.

The third embodiment will now be described in detail.

The scanning laser retinal function camera 103 has two separate laserbeam sources, a first laser source 26 producing infrared laser light inthe region of 758 nm (700 nm–805 nm), and a second laser source 27producing infrared laser light in the region of 910 nm (805 nm–1000 nm).Infrared laser beams from the first and second sources pass throughrespective electrooptic modulators 261, 271, which provide individualintensity control of the respective infrared beams. An optical beamadder 28 located to receive laser beams emergent from the respectiveelectrooptic modulators allows both infrared laser beams access to anoptical axis of the scanning laser retinal function camera. This allowsthe infrared laser sources sequentially to illuminate a retina at thefocal plane of an eye 10. The laser beam from the adder 28 passesthrough a focus lens 31, which allows the laser beam to be focused onthe retina. The light from the focus lens 31 is reflected by a mirror 50and passes through a central transmission hole 81 in a perforated mirror80 onto a rotating eighteen-facet polygonal mirror 150 rotatable atabout 52,100 rpm. (For clarity, a 12-facet polygonal mirror 150 isillustrated). A half-silvered mirror may be used in place of theperforated mirror 80, and such a half-silvered mirror may be constructedto have a greater reflectance of light than transmission of light. Therotatable polygonal mirror 150 reflects the infrared laser light beamonto a concave mirror 160 as a linear horizontal scan with a repetitionrate of 15,625 Hz corresponding to a closed circuit television standard,in a horizontal axis of the eye 10. The concave mirror 160 reflects andfocuses the infrared laser light beam onto a movable galvanometer mirror170. The galvanometer mirror 170 is electrically movable so as to varythe reflection angle to produce a vertical scan with a repetition rateof 50 Hz in a vertical axis of the eye 10. The infrared laser light isreflected by the galvanometer mirror 170 to a concave mirror 180, whichfocuses the infrared laser light as a 10 micron diameter spot onto thefocal plane of the eye 10.

It will be understood that alternative apparatus for producing ascanning beam may be used, for example, a vibrating mirror andgalvanometer two-axis scanner may replace the polygonal mirror andgalvanometer scanner.

Light reflected from the retina returns along the same pathway and isde-scanned by the galvanometer mirror 170 and the rotating polygonalmirror 150. The reflected light is then reflected by perforated mirror80, or a half-silvered mirror, towards a focussing lens 120 andsequentially through an occluding diaphragm 110 and onto an infrareddetector such as an avalanche photodiode and processor 141. The returnsignals are detected on a pixel-by-pixel basis and then transferred to aframe grabber card (not shown) to construct a synchronised data frame.The data frame is synchronised to the vertical frame, horizontal lineand pixel clock signals from the scanning apparatus. A controller (notshown), that receives horizontal and vertical synchronising signals fromthe scanning apparatus, activates and deactivates the laserssequentially.

The digital image processing is performed by a computer using digitalimage processing software such as Matlab™ available from The MathWorksInc, 3 Apple Hill Drive, Natick Mass. 01760-2089, United States andLabview™ available from National Instruments Corporation, Austin Tex.,United States. The isoreflective point is defined. The flicker contrastof each pixel, to the isoreflective point, is determined. A falsecolour, red or blue, is allocated to each pixel and the intensity ofcolour related to flicker contrast. The isoreflective point is allocatedyellow. A composite image is constructed by combining the red, yellowand blue image data.

The scanning laser retinal function camera illustrated in FIG. 3 issimilar to the Digital 35 Laser Scanning Fundus Camera, described byPlesch et al. in Applied Optics, Vol. 26, No. 8, page 1480–86, Apr. 15,1987. This device-uses a collimated laser beam focussed by the eye to aspot of 10–15 microns diameter for illumination of a single point of theretina. The light scattered back from the retina, normally 3–5% of theincident light, is collected through the outer 95% of the pupil. Angularscanning of the illuminating laser beam sweep the spot across the retinaand results in time resolved sequential imaging of the retina. Thedevice is connected to a digital image buffer and a microcomputer forimage storage and processing.

The scanning laser retinal function camera illustrated in FIG. 3contains an optical beam adder that is used in U.S. Pat. No. 6,099,127.This uses a red 670 nm, green 540 nm and blue 488 nm laser lightsources, with an optical beam adder, which separately illuminate thefundus. The three images obtained are used to construct a colourrepresentation of the fundus.

Optimising the Retinal Function Image

The ideal retinal function image contains:

-   -   a stable non-flickering background of light reflected from        retinal and choroidal pigments and cells;    -   maximal contrast of light from deoxygenated and oxygenated        blood; and    -   similar depth of infrared light retinal penetration to image the        same retinal and choroidal components.

The retinal function image may be time synchronised with an R wave of apatient's electrocardiogram, to allow retinal metabolism to be studiedat different phases of the cardiac cycle. The data frame from which theretinal function image is constructed by the digital image processorconsists of data from (in this example) the 512 scan lines each scannedwith, at least, two wavelengths of light. The data frame “starts” atline 1 and “ends” at line 512. Alternatively, the R wave of theelectrocardiogram may be used as a time signal to define the next linescan x as the start of the R wave synchronised data frame. The retinalfunction image is then constructed by the digital processor from a dataframe defined by line x to line 512 and the next line 1 to line x−1. Atime delay may be selected by the operator between the R waveelectrocardiogram time signal and scan line x.

Infrared light in the region of 758 nm and 1000 nm may provide maximumcontrast for haemoglobin oxygenation. In order to optimise thefunctional retinal image the 1000 nm wavelength illumination may need tobe tuned closer to 805 nm. A retinal background isoreflectivenon-flickering point will be determined when the light energy reflectedfrom retinal and choroidal pigment cells by the 700 nm–805 nm and 805nm–1000 nm sources is equal. This will provide a stable non-flickeringbackground on which to contrast the functional image. The light sourcewavelength or power may be variable to allow tuning of the image.

The individual light source intensity may be controlled by altering thesupply power using a current limiting technology. Alternatively thelight output may be controlled with either a variable aperture diaphragmor an electrooptic modulator.

Alternative Light Wavelengths

Alternative wavelengths of the light spectrum where there aresignificant differences of light absorption between oxyhaemoglobin anddeoxyhaemoglobin may be used to generate the functional image. Lightbetween 488 nm and 1000 nm may be used. Suitable wavelengths are 488 nmand 600 nm, 630 nm, 635 nm, 670 nm, or 760 nm; 635 nm and 830 nm or 910nm; 670 nm and 830 mm or 910 nm; 760 nm and 830 nm or 910 nm. Thevisible light, between 488 nm and 700 nm, would have less retinal tissuepenetration than the near infrared light.

Alternative Imaging Technology

Alternative imaging arrangements may be used to generate the separatewavelength retinal and choroidal images needed to generate thefunctional retinal image. The scanning laser retinal function camera mayhave a confocal filter positioned upstream of the detector to allow theretinal surface image to be detected while blocking the deeper choroidalimage. A confocal filter is used to generate optical sections. Ananti-confocal filter may be positioned upstream of the detector to blockthe retinal surface image and allow a deeper choroidal image to bedetected.

A three dimensional retinal metabolic image may be formed by integratinga series of confocal optical sections obtained at sequential depths.

The scanning laser retinal function camera may have an orthogonalpolarising filter positioned upstream of the detector. The function ofthe orthogonal polarising filter is to block the surface reflected lightthat has the same linear polarisation as the laser illumination light.This will allow orthogonally polarised light that has been scattered andreflected from deeper layers to be detected and form the image. Thepolarising filters therefore improve contrast by blocking surfacereflections.

The retinal function camera may have a linear polarising filter on theilluminating axis and an orthogonal polarising filter on the imagingaxis. The orthogonal polarising filter will block the reflected lightfrom the retinal surface that has the same linear polarisation as theillumination light. This will allow light from deeper layers, thatcontain haemoglobin oxygenation information, to be detected and form theimage.

The scanning laser retinal function camera controller sequentiallyactivates the different wavelength lasers, which the scanner scansacross a portion of the retina to form a complete data frame with eachlaser. Preferably, each scan line, rather than each frame, may bescanned twice, with alternate wavelengths, to minimise movementartefacts between two retinal images. The portion of the retina scannedsequentially, with alternate wavelengths, may be less than a scan line,such as a pixel, or it may be greater than a scan line, such as a flame.That is, the portion of a retina scanned with each alternate wavelengthmay correspond to a pixel, a line scan or a frame in the image formedfrom the scans. Sequentially scanning each line twice maximises thesignal to noise ratio while minimising retinal light exposure.

Alternatively, a portion of the retina scan, such as sequential scanlines may be illuminated. This would form an interlaced data frame. Thedata frame would need to be deinterlaced to form the two separate imagesprior to constructing the retinal function image.

Simultaneous illumination of the retina with two separate wavelengths oflight is possible by either using the integrating sphere or a dichroicbeam combiner. The separate wavelength images may be obtained with adichroic beam splitter and separate imaging optics.

It will be understood that although the invention has been described inrelation to a retinal function camera, the invention has equalapplicability to, for example, the use of a confocal microscope ormultispectral camera for imaging.

1. A retinal function camera comprising: a first source of light of afirst wavelength band; a second source of light of a second wavelengthband, the absorptivity of light of the first wavelength band byoxygenated blood being greater than the absorptivity of light of thesecond wavelength band and the absorptivity of light of the firstwavelength band by deoxygenated blood being less than the absorptivityof light of the second wavelength band; means for focusing lightselectively from the first and second sources on a portion of a retinaof an eye; imaging means for producing respective images of a portion ofthe retina illuminated with the respective wavelength bands; andprocessing means adapted to process the respective images obtained bythe imaging means to determine isoreflective points of the respectiveimages at which absorption of light of the first wavelength issubstantially equal to absorption of light of the second wavelength andareas of the respective images having differential absorptivity for thefirst and second wavelengths, to obtain a retinal function image basedon haemoglobin oxygenation.
 2. A retinal function camera as claimed inclaim 1, wherein the processing means comprises means for displaying therespective images alternately, at a predetermined frequency, such thatthe areas of the respective images having differential absorptivity atthe first and second wavelengths, flicker.
 3. A retinal function cameraas claimed in claim 2, wherein the predetermined frequency is 12 Hz. 4.A retinal function camera as claimed in claim 1, wherein the first andsecond wavelength bands are selected between 488 nm and 1000 nm, toproduce a functional image.
 5. A retinal function camera as claimed inclaim 1, wherein the first wavelength band is centred substantially on830 nm and the second wavelength band is centred substantially on one of635 nm and 670 nm and 760 nm; or the first wavelength band is centredsubstantially on 910 nm and the second wavelength band is centredsubstantially on one of 635 nm and 670 nm and 760 nm.
 6. A retinalfunction camera as claimed in claim 1, wherein there is provided anarray of superluminescent diodes producing light in the wavelength band550 nm to 650 nm to produce a conventional image and the secondwavelength band is 700 nm to 805 nm and the first wavelength band is 805nm to 1000 nm to produce a functional image.
 7. A retinal functioncamera as claimed in claim 6, wherein the processing means comprisesmeans for assigning the respective images created with the firstwavelength band and the second wavelength band and the conventionalimage with false colours respectively and combining the three images toform a combined colour image.
 8. A retinal function camera as claimed inclaim 1, wherein the first source of light is a laser.
 9. A retinalfunction camera as claimed in claim 8 further comprising an optical beamadder for allowing a first laser beam from the first light source and asecond laser beam from the second light source access to an opticalaccess of the retinal function camera.
 10. A retinal function camera asclaimed in claim 1, wherein the second source of light is a laser.
 11. Aretinal function camera as claimed in claim 1, wherein the first andsecond sources of light are superluminescent diodes provided with narrowband pass filters to restrict the waveband of light emitted.
 12. Aretinal function camera as claimed in claim 1, wherein a wide spectrumlight source is provided emitting wavelengths from near infrared throughthe visible spectrum and the first and second light sources are producedby passing the wide spectrum light through narrow band pass filters. 13.A retinal function camera as claimed in claim 1, wherein the means forfocusing light selectively from the first source and the second sourcecomprise means for focusing light from the first and second sources andfor sequentially switching on and off the first and second sourcesalternately.
 14. A retinal function camera as claimed in claim 1,wherein the means for focusing light selectively from the first andsecond sources comprise means for focusing light from the first andsecond sources and shutter means for alternately interrupting light fromthe first and second sources, respectively.
 15. A retinal functioncamera as claimed in claim 1, wherein the processing means includesmeans for comparing an image with a reference image formed at an earliertime.
 16. A retinal function camera as claimed in claim 15, wherein theprocessing means includes pattern recognition means for aligning theimage with the reference image.
 17. A retinal function camera as claimedin claim 1, wherein the processing means includes means for assigningfirst areas of the respective images having differential absorptivityfor the first and second wavelengths corresponding to portions of theretina having greater oxygenation than the isoreflective points a firstfalse colour and for assigning second areas of the respective imageshaving differential absorptivity for the first and second wavelengthscorresponding to portions of the retina having less oxygenation than theisoreflective points a second false colour and for generatingintensities of the false colours at each point in the image proportionalto the difference in oxygenation of that respective point from theoxygenation of the isoreflective points.
 18. A retinal functional cameraas claimed in claim 17, wherein one of wavelength and power of at leastone of the first source and the second source is variable to tune theisoreflective point.
 19. A retinal function camera as claimed in claim1, wherein the processing means includes means for calibratingoxygenation by identifying a portion of the retina image of maximumoxygenation and a portion of the retina image of maximum deoxygenation.20. A retinal function camera as claimed in claim 1, wherein the meansfor focusing light include scanning means for scanning the focused lightacross at least a portion of the retina.
 21. A retinal function cameraas claimed in claim 20, wherein the scanning means include firstscanning means for scanning the focused light horizontally across the atleast a portion of the retina and second scanning means for scanning thefocused light vertically across the at least a portion of the retina.22. A retinal function camera as claimed in claim 21, wherein the secondscanning means includes a galvanometer scanner.
 23. A retinal functioncamera as claimed in claim 21, wherein first synchronising means areprovided to synchronise the first and second scanning means withselection means for selectively operating the first source of light andthe second source of light.
 24. A retinal function camera as claimed inclaim 21, wherein second synchronising means are provided to synchronisethe first and second scanning means with the imaging and processingmeans.
 25. A retinal function camera as claimed in claim 21, wherein thefirst and second scanning means are adapted to de-scan light reflectedfrom the retina and reflecting de-scanned light to the imaging andprocessing means.
 26. A retinal function camera as claimed in claim 21,wherein the first and second scanning means operate at frequenciescorresponding to television scanning frequencies such that the imagingand processing means may be used to form a television image.
 27. Aretinal function camera as claimed in claim 20, wherein the firstscanning means includes one of a rotatable polygonal mirror and avibratable plane mirror.
 28. A retinal function camera as claimed inclaim 20, wherein the processing means includes means for assigningfirst areas of the respective images having differential absorptivityfor the first and second wavelengths corresponding to portions of theretina having greater oxygenation than the isoreflective points a firstfalse colour and for assigning second areas of the respective imageshaving differential absorptivity for the first and second wavelengthscorresponding to portions of the retina having less oxygenation than theisoreflective points a second false colour and for generatingintensities of the first and second false colours at each point in theimage proportional to the difference in oxygenation of that respectivepoint from the oxygenation of the isoreflective points.
 29. A retinalfunction camera as claimed in claim 1 further comprising a confocalfilter locatable upstream of the imaging and processing means fordetecting a retinal surface image and for blocking a deeper choroidalimage and an anti-confocal filter locatable upstream of the imaging andprocessing means to block the retinal surface image and allow the deeperchoroidal image to be detected.
 30. A retinal function camera as claimedin claim 1 wherein a first linear polarising filter is provided betweenthe light source and the eye and a second linear polarising filterorthogonal to the first linear polarising filter is provided between theeye and the imaging and processing means such that the second orthogonalpolarising filter blocks light reflected from a surface of the eye. 31.A retinal function camera as claimed in claim 1, wherein the processingmeans includes an imaging device sensitive to light emitted by the firstsource of light and the second source of light.
 32. A retinal functioncamera as claimed in claim 31, wherein the imaging device is one of aCMOS array, a CCD array, a photodetector or an infrared image sensor.33. A retinal function camera as claimed in claim 1, further comprisingoptical fibre and lens means for producing a point source of light fromlight from the first source of light and light from the second source oflight.
 34. A retinal function camera as claimed in claim 1, furthercomprising a confocal filter locatable upstream of the imaging andprocessing means to allow the retinal surface image to be detected whileblocking the deeper choroidal image.
 35. A retinal function camera asclaimed in claim 1 further comprising an anti-confocal filter locatableupstream of the imaging and processing means to block the retinalsurface image thereby allowing a deeper choroidal image to be detected.36. A retinal function camera as claimed in claim 1 whereinsynchronising means are provided to synchronise the imaging andprocessing means with an R wave of a patient's electrocardiogram, toallow retinal metabolism to be studied at different phases of thecardiac cycle.
 37. A method of obtaining a retinal function image basedon haemoglobin oxygenation the method comprising the steps of: a)providing a retinal function camera having a first source of light of afirst wavelength band and a second source of light of a secondwavelength band, the absorptivity of light of the first wavelength bandby oxygenated blood being greater than the absorptivity of light of thesecond wavelength band and the absorptivity of light of the firstwavelength band by deoxygenated blood being less than the absorptivityof light of the second wavelength band; b) focusing light selectivelyfrom the first and second sources on a portion of a retina of an eye; c)producing respective images of the portion of the retina illuminatedwith the respective wavelength bands; and d) processing the respectiveimages to determine isoreflective points of the respective images atwhich absorption of light of the first wavelength is substantially equalto absorption of light of the second wavelength and areas of therespective images having differential absorptivity for the first andsecond wavelengths to obtain a retinal function image based onhaemoglobin oxygenation.
 38. A method as claimed in claim 37, whereinthe step of processing the images comprises, assigning to first portionsof the image corresponding to portions of the retina having greateroxygenation than the isoreflective point a first false colour andgenerating intensity of the false colours at each point in the imageproportional to the flicker contrast that is proportional to thedifference in oxygenation of that respective point from the oxygenationof the isoreflective point, assigning to second portions of the imagecorresponding to portions of the retina having less oxygenation than theisoreflective point a second false colour and generating intensity ofthe false colours at each point in the image proportional to the flickercontrast that is proportional to the difference in oxygenation of thatrespective point from the oxygenation of the isoreflective point,assigning a third colour to the isoreflective point, and constructing acomposite image by combining the first, second and third colour imagedata to form a colour image of retinal function based on haemoglobinoxygenation data.
 39. A method as claimed in claim 37, wherein the firstcolour is red.
 40. A method as claimed in claim 37, wherein the secondcolour is blue.
 41. A method as claimed in claims 37, wherein the thirdcolour is yellow.
 42. A retinal function camera comprising: a firstsource of light of a first wavelength band; a second source of light ofa second wavelength band, the absorptivity of light of the firstwavelength band by oxygenated blood being greater than the absorptivityof light of the second wavelength band and the absorptivity of light ofthe first wavelength band by deoxygenated blood being less than theabsorptivity of light of the second wavelength band; means for focusinglight selectively from the first and second sources on a portion of aretina of an eye; imaging means for producing respective images of aportion of the retina illuminated with the respective wavelength bands;and processing means adapted to process the respective images obtainedby the imaging means to determine isoreflective points of the respectiveimages at which absorption of light of the first wavelength issubstantially equal to absorption of light of the second wavelength andareas of the respective images having differential absorptivity for thefirst and second wavelengths, to obtain a retinal function image basedon haemoglobin oxygenation, wherein the processing means comprises meansfor displaying the respective images alternately, at a predeterminedfrequency, such that the areas of the respective images havingdifferential absorptivity at the first and second wavelengths, flicker.43. A method of obtaining a retinal function image based on haemoglobinoxygenation the method comprising the steps of: a) providing a retinalfunction camera having a first source of light of a first wavelengthband and a second source of light of a second wavelength band, theabsorptivity of light of the first wavelength band by oxygenated bloodbeing greater than the absorptivity of light of the second wavelengthband and the absorptivity of light of the first wavelength band bydeoxygenated blood being less than the absorptivity of light of thesecond wavelength band; b) focusing light selectively from the first andsecond sources on a portion of a retina of an eye; c) producingrespective images of the portion of the retina illuminated with therespective wavelength bands; and d) processing the respective images todetermine isoreflective points of the respective images at whichabsorption of light of the first wavelength is substantially equal toabsorption of light of the second wavelength and areas of the respectiveimages having differential absorptivity for the first and secondwavelengths to obtain a retinal function image based on haemoglobinoxygenation; wherein the step of processing the images comprises,assigning to first portions of the image corresponding to portions ofthe retina having greater oxygenation than the isoreflective point afirst false colour and generating intensity of the false colours at eachpoint in the image proportional to a flicker contrast that isproportional to the difference in oxygenation of that respective pointfrom the oxygenation of the isoreflective point, assigning to secondportions of the image corresponding to portions of the retina havingless oxygenation than the isoreflective point a second false colour andgenerating intensity of the false colours at each point in the imageproportional to the flicker contrast that is proportional to thedifference in oxygenation of that respective point from the oxygenationof the isoreflective point, assigning a third colour to theisoreflective point, and constructing a composite image by combining thefirst, second and third colour image data to form a colour image ofretinal function based on haemoglobin oxygenation data.